The present invention relates to an alkaline storage battery, particularly to an improvement in a separator used therein.
In recent years, alkaline storage batteries such as nickel-cadmium storage batteries and nickel-metal hydride storage batteries are attracting an attention as a power source for driving an electric vehicle since these batteries are expected to have a high output voltage, a high energy density and a long cycle life. Therefore, there increases a demand for further improvement in the performance of such alkaline storage batteries.
As one countermeasure for improving the performance of the alkaline storage batteries, a separator to separate a positive electrode from a negative electrode is investigated. For the separators in the alkaline storage batteries, a non-woven fabric composed of fibers made of an olefin resin such as polypropylene or an amide resin has been used. The separator made of the amide resin demonstrates a high liquid-absorbing property since the amide resin has a high hydrophilic property. However, the amide resin is low in alkali-resistance and heat-resistance. Therefore, the separator of the amide resin can not be stable for a long period at a high temperature which is assumed in a case when the battery is applied in an electric vehicle.
For that reason, investigations are mainly devoted to an improvement in the separator made of the olefin resin which is excellent in mechanical strength, alkali-resistance and heat-resistance. However, the separator made of the olefin resin demonstrates low liquid-absorbing property for an electrolyte since the olefin resin has low hydrophilic property. Therefore, various methods have been proposed for increasing the hydrophilic property and the liquid-absorbing property of the separator, thereby improving life characteristics and discharge characteristics of the battery.
For instance, according to Japanese Laid-Open Patent Publication Sho 62-115657 or Japanese Laid-Open Patent Publication Sho 64-57568, hydrophilic property of a separator made of an olefin resin poor in hydrophilic property is improved by a sulfonation treatment in which the separator is immersed in a concentrated sulfonic acid. However, the sulfonation treatment has a problem of lowering a mechanical strength of the separator because the treatment results in cutting of the carbon-carbon bonds in the resin.
Further, Japanese Laid-Open Patent Publication Hei 7-130392 proposes a method for improving a hydrophilic property of a separator by applying a surfactant which contain s sulfone groups to the separator. Although alkaline storage batteries configured with such separators demonstrate favorable characteristics at room temperature, they cannot have satisfactory life characteristic at high temperature because the surfactant gradually detaches from the surface of the separator.
Incidentally, another means for improving the discharge characteristic of an alkaline storage battery is investigated in making a separator thinner. The decreased thickness of the separator affects decreased electric and physical resistances between a positive electrode and a negative electrode, and also affects an increased proportion of a space occupied by the electrodes which participates in an electrochemical reaction of the battery. However, in a case of making the separator thinner by simply decreasing an amount or pieces of filaments per unit area (that is so called "METSUKE"), the liquid-absorbing property of the separator for an electrolyte is lowered, and the capacity of the battery is also decreased. In addition, since the mechanical strength of the separator is lowered, the separator is made liable to be damaged at the time of assembling the battery. Short-circuiting between the electrodes is also made liable to occur after assembling. In a case of making a diameter of the filament small while maintaining the pieces of the filaments to that of the conventional one, it is possible to reduce the thickness of the separator while maintaining the liquid-absorbing property for the electrolyte and the short-circuit resistance. However, there increases a hazard of lowering the mechanical strength and damaging the separator.